Derive a link's description from its title

Previously, a LinkDescriptor had to be created with both a rel and a
description. If a description was not provided a failure would occur.

This commit relaxes the above-described restriction by allowing a
link's title to be used as its default description. If a descriptor
has a description, it will always be used irrespective of whether or
not the link has a title. If the descriptor does not have a
description and the link does have a title, the link's title will be
used. If the descriptor does not have a description and the link does
not have a title a failure will occur.

Closes gh-105

spring-restdocs-core/src/test/resources/link-payloads/atom/multiple-links-different-rels.json

@@ -1,7 +1,8 @@
 {
 	"links": [ {
 		"rel": "alpha",
-		"href": "http://alpha.example.com"
+		"href": "http://alpha.example.com",
+		"title": "Alpha"
 	}, {
 		"rel": "bravo",
 		"href": "http://bravo.example.com"

spring-restdocs-core/src/test/resources/link-payloads/atom/multiple-links-same-rels.json

@@ -1,7 +1,8 @@
 {
 	"links": [ {
 		"rel": "alpha",
-		"href": "http://alpha.example.com/one"
+		"href": "http://alpha.example.com/one",
+		"title": "Alpha one"
 	}, {
 		"rel": "alpha",
 		"href": "http://alpha.example.com/two"

spring-restdocs-core/src/test/resources/link-payloads/atom/single-link.json

@@ -1,6 +1,7 @@
 {
 	"links": [ {
 		"rel": "alpha",
-		"href": "http://alpha.example.com"
+		"href": "http://alpha.example.com",
+		"title": "Alpha"
 	} ]
 }

spring-restdocs-core/src/test/resources/link-payloads/hal/multiple-links-different-rels.json

@@ -1,7 +1,8 @@
 {
 	"_links": {
 		"alpha": {
-			"href": "http://alpha.example.com"
+			"href": "http://alpha.example.com",
+			"title": "Alpha"
 		},
 		"bravo": {
 			"href": "http://bravo.example.com"

spring-restdocs-core/src/test/resources/link-payloads/hal/multiple-links-same-rels.json

@@ -1,7 +1,8 @@
 {
 	"_links": {
 		"alpha": [{
-		    "href": "http://alpha.example.com/one"
+		    "href": "http://alpha.example.com/one",
+			"title": "Alpha one"
 		}, {
 		    "href": "http://alpha.example.com/two"
 		}]

spring-restdocs-core/src/test/resources/link-payloads/hal/single-link.json

@@ -1,7 +1,8 @@
 {
 	"_links": {
 		"alpha": {
-		    "href": "http://alpha.example.com"
+		    "href": "http://alpha.example.com",
+			"title": "Alpha"
 		}
 	}
 }